1. Field of the Invention
The present invention relates to rotor position detection of switched reluctance motors, and more particularly to a position detection apparatus for a switched reluctance motor wherein a single sensor is used for detecting a rotor position and thus driving the motor.
2. Description of the Prior Art
FIG. 1 is a schematic plan view of a general switched reluctance motor including six stators and four rotors. FIG. 2 is a schematic plan view of a general sensing disc including three sensors. As shown in the figures, the motor comprises a stator including six fixed magnetic poles 3a uniformly spaced from one another at an angle of 60.degree. and fixedly mounted to a motor housing. Coils 4 are wound around one of pairs a-a', b-b' and c-c' of opposed fixed magnetic poles 3a to form magnetic poles with reversed polarity. With this arrangement, the pairs of opposed magnetic poles 3a have different phases a, b and c, respectively. The reluctance motor also comprises a rotor 2 disposed inwardly of the stator 3 to be rotatable about a rotating shaft 1 and provided with four rotating magnetic poles 4a spaced uniformly from one another at an angle of 90.degree..
In such a conventional switched reluctance motor, there is also provided a sensing disc 5 for detecting a position of the rotor 2. As shown in FIG. 2, the sensing disc 5 has four protrusions 5a each having a width corresponding to 30.degree. and four openings 5b each defined between adjacent protrusions 5a and having a width corresponding to 60.degree.. The sensing disc 5 is fixedly mounted to the rotating shaft 1, so as to be rotatable about the rotating shaft 1, together with the rotor 2. For detecting a rotated position of the sensing disc 5, three sensors S1, S2 and S3 are arranged on a circle and uniformly spaced from one another at an angle of 120.degree.. Based on position detection signals from the sensors S1, S2 and S3, electric power is sequentially supplied to respective coils 4 of phases a, b and c wound around the fixed magnetic poles 3a, so as to drive the motor.
Rotor position detection and associated operations of the conventional switched reluctance motor with the above-mentioned construction will be now described, in conjunction with FIG. 3.
As the motor rotates counter-clockwise, the rotor 2 and sensing disc 5 rotate in the same direction. At this time, the sensors S1, S2 and S3 detect the rotated position of the sensing disc 5, As shown in FIG. 3A, the first, sensor S1 is maintained at its OFF state and thus outputs a low level signal, for 30.degree. corresponding to the angle through which each protrusion 5a passes. Thereafter, the first sensor S1 is maintained at its ON state and thus outputs a high level signal, for 60.degree. corresponding to the angle through which each opening 5b passes. These operations of the first sensor S1 are repeated. Similarly, the second sensor S2 and the third sensor S3 repeat their operations of being maintained at their OFF states for 30.degree. and then maintained at their ON states for 60.degree.. Such operations of the sensors S1, S2 and S3 are carried out at intervals of 30.degree..
As the sensors S1 and S2 detect the rotated positions of rotor 2, they generate detection signals indicative of the rotated position of rotor 2. These detection signals are logically combined so as to drive sequentially the phases a, b and c. As shown in FIGS. 3D, 3E and 3F, the two detection signals are combined to obtain signals /S1.S2, S1./S2 and S1.S2, by which the phases a, b and c are driven.
FIGS. 3D, 3E and 3F are waveform diagrams of driving signals for the phases a, b and c when the motor rotates in counter-clockwise, respectively. These driving signals are obtained by the combination of two detection signals S1 and S3. When the motor rotates clockwise, detection signals from the sensors S1 and S3 are combined to obtain driving signals for driving respective phases a, b and c. Accordingly, at least two sensors are needed for rotating the motor in one direction. Where the motor is desired to rotate in both directions, a phase advance angle should be considered. With only two sensors, the advance angle may not be accurately detected. Therefore, it is required to use at least three sensors, for driving the motor in both directions.
Such a requirement of using a plurality of sensors (generally, three in a 6-4 pole type) for position detection in the above-mentioned position detection apparatus causes complicated mounting and wiring. Such a complication in mounting and wiring results in a degradation in productivity and an increase in cost. The mounting of many sensors causes an error in relative position among sensors. As a result, there is a drawback that position detection signals may have incorrect waveforms.